The present invention is directed to a process for the production of natural gas condensate, and specifically to a process for the production of natural gas condensate having a reduced amount of mercury from a mercury-containing natural gas wellstream.
Natural gas which is produced from a natural gas well is typically separated and purified to provide products for a variety of end uses. The high-pressure mixture produced from the well, i.e. the wellstream, is typically sent to a separator vessel or a series of separator vessels maintained at progressively lower pressures where the wellstream is separated into a gaseous fraction and a liquid fraction.
The gaseous fraction leaving the separator, which may contain the impurities mercury, carbon dioxide and hydrogen sulfide, is sent to a gas treatment and purification plant where, typically, the mercury concentration is reduced to &lt; 0.1 micrograms/m.sup.3, the CO.sub.2 concentration is reduced to the parts per million (ppm) level, and the H.sub.2 S to about one (1) ppm.
The liquid fraction is typically preheated, e.g. to about 150.degree. C., and is then sent to a stabilizer column. In the upper section of the stabilizer column, the stream is rectified, i.e., the heavy hydrocarbons are removed from the vapor phase, and in the lower section of the stabilizer column, the liquid stream is stripped of light hydrocarbon components. Complete stabilization can be further enhanced by heating the bottom liquid stream of the stabilizer column in a reboiler. The reboiler supplies additional heat in order to reduce the light hydrocarbon content of the liquid. The stabilizer column produces two streams: a stream which leaves the top of the stabilizer column containing gaseous components, e.g. CO.sub.2, H.sub.2 S, etc., and low molecular weight hydrocarbons, e.g. C.sub.1 -C.sub.4 and a stabilized condensate stream which leaves the bottom of the stabilizer column.
The purification of the gaseous fraction is commonly achieved by passing the gaseous fraction over a bed of activated carbon which has been impregnated with sulfur. In this step, the mercury in the gas reacts with the sulfur and is essentially removed from the gaseous fraction. Typically, the mercury content of the gas can be reduced from about 250 micrograms/m.sub.3 or higher to less than about 0.1 micrograms/m.sub.3.
The gas leaving the sulfur/carbon bed is then treated with a hot aqueous potassium carbonate solution which has the ability to absorb CO.sub.2 and H.sub.2 S. This step produces two streams, a rich carbonate solution and a natural gas stream having a reduced CO.sub.2 and H.sub.2 S content. For example, the CO.sub.2 content of the gas can be reduced from about 15% to about 0.3% and the H.sub.2 S content from about 80 ppm to about 6 ppm.
The rich carbonate solution may then be sent to a regenerator where it is heated to a sufficiently high temperature, e.g. 120.degree. C., in order to expel the CO.sub.2 and H.sub.2 S, permitting the carbonate solution to be recycled. The CO.sub.2 and H.sub.2 S mixture expelled from the rich carbonate solution is cooled and this stream, known as the "acid gas" stream, is sent for disposal.
The natural gas stream which resulted from treatment with the carbonate solution is further treated in order to reduce the amount of CO.sub.2 and H.sub.2 S by heating the gas with an aqueous solution of diethanolamine. Diethanolamine has the ability to absorb CO.sub.2 and H.sub.2 S and can typically reduce the CO.sub.2 content from about 0.3% to about 50 ppm, and the H.sub.2 S content from about 6 ppm to about 1 ppm. The resulting hydrocarbon gas is then generally considered sufficiently pure for industrial or domestic use.
It has been found that the mercury in wellstreams from gas producing wells which contain mercury is partitioned among the gaseous and liquid streams. This mercury is thought to originate from the geologic deposits in which the natural gas is entrapped. It will also be appreciated by those skilled in the art that trace amounts of nickel, vanadium, salt, moisture and sediment are typically present in the liquid fraction treated in accordance with the present invention.
Typical steps for the processing of the liquid fraction of the wellstream do not reduce the amount of mercury in the liquid fraction leaving the separator. For example, a liquid fraction leaving the separator having a mercury content of about 220 .mu.g/kg (ppb) will yield a stabilized condensate containing about 220 .mu.g/kg (ppb). The presence of mercury in a natural gas condensate is undesirable and can cause damage to downstream processing equipment.
Equipment damage may result when mercury accumulates in equipment constructed of various metals, especially aluminum, by forming an amalgam with the metal. For example, in the production of ethylene, cracked natural gas condensate is commonly passed through a heat exchanger constructed of aluminum. Such equipment exists in the section of the ethylene manufacturing facility where ethylene is separated from hydrogen, ethane and other hydrocarbons by chilling. It has been found that mercury tends to amalgamate with the aluminum of which the heat exchanger is constructed, thereby creating the risk of corrosion cracking with potentially catastrophic results.